Transcript Muon Anomalous Magnetic Moment --a harbinger of new physics Chang Liu Physics 564

Muon Anomalous Magnetic Moment
--a harbinger of new physics
Chang Liu
Physics 564
Outline
 Introduction
 Experimental Approach
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•
Generate and Store Muons
Measure Anomalous Spin Precession Frequency
 Theoretical Approach
•
•
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QED
Hadronic
Electroweak
 Comparison
 New Physics?
Anomalous Magnetic Moment
• Spin Angular Momentum:
Dirac Equation predicted
for spin-1/2 particle w/o substructure
--1928
• However,
 Proton
 Neutron
Evidence of Quark Model!
--1960s
Definition of Anomalous Magnetic Moment:
Which can be theoretically calculated very accurate!
Thus it is a good choice for experimentalists to examine theoretical correction.
Brookhaven E821 Experiment
Anomalous spin precession frequency:
Larmor frequency:
Anomalous magnetic moment:
Both frequencies are measured in units of the the free proton precession frequency (by NMR).
Blind Data Analysis Philosophy:
Independent analyses of the two parameters were made by different groups.
No one knows information to compute the final result before completion.
Generate and Store Muons
Protons+ nickel target
pions
secondary muons
The charge of muons can be selected by adjusting experimental parameter.
Muons were polarized and injected to the muon storage ring.
Inflector and Storage Ring
Measure The Anomalous Magnetic Moment
• Muons will decay in the storage ring
• Energy of the electron is related to the polarized direction of muon.
•
•
If spin is aligned with momentum, it’s LESS likely to get high-energy electron.
If spin is opposite to momentum, it’s MORE likely to get high-energy electron.
• The rate of detected electrons above a specified threshold energy:
If we can get the time spectrum of
detected electrons, then we can get
anomalous spin precession
frequency!
Need to Measure:
electron energy
as following…
Electron Detector System
Top view of the chamber and calorimeter.
The energy of the decay positron was detected by the calorimeter.
Data Analysis
Pile-Up
Pulses separated by >3.5-5ns
are resolved reliably and
recorded as individual events.
One Pulse
Two Close Pulses
The injected ~50-ns-long muon bunches expand all over the ring after about
0.025ns.
Pileup-subtracted histograms are created for each detector and run and now are
ready for fitting.
Data Analysis
Histogram of 4 billion decay electrons from the 2000 run.
In principle, these data can be fitted to the function we
introduced previously.
In fact, more data analysis techniques are used.
Muon loss events vs. time after injection.
Theory
The first Order is:
There are also other known or unknown contributions:
QED, Hadronic, Electroweak, and so on.
Need to Calculate higher order Feynman Diagram.
QED Contribution
Second order diagrams
Hadronic and Electroweak Contribution
Hadronic Effects:
Electroweak Effects:
Comparison
Experimental value:
QED:
Hadronic:
Electroweak:
Standard Model Value:
Room for new physics:
New Physics?
 IF THE NEW PHYSICS IS,
• SUperSYmmetry (SUSY)
New SUSY particles are expected to be discovered soon at Fermilab and LHC.
• Minimal SUperGRAvity (mSUGRA)
The branching ratio of
will be largely enhanced and can be detected by Tevatron.
• Anomalous W boson properties
Not so likely, it have been ruled out because of contradiction to existing data.
• New gauge bosons, muon compositeness, extra Higgs bosons,
leptoquarks ,bileptons, compact extra dimensions,
• And many many…
The parameters of new physics can be narrowed and determined with the development of
more accurate experiments for muon anomalous magnetic moment and other effects.